It is well known to use fluid pressure regulators in irrigation systems in order to provide constant outlet pressure over a wide range of inlet pressures. The need for such regulators is particularly acute in low pressure systems because a slight variation in pressure along the system operating at low pressure causes a much greater variation in discharge than the same system operating at high pressure.
The assignee of this invention currently manufactures and sells fluid pressure regulators of the flow through type, having an inlet at one end of a tubular housing and an outlet at the other end of the tubular housing. A plunger seat is secured within the housing and is adapted to be engaged by a lower end of a tubular plunger assembly which is spring biased away from the seat (in the direction of fluid flow) so that under low pressure conditions, maximum flow through the regulator is permitted. In the event of excess pressure, the plunger is moved back by pressure within the diaphragm chamber (above the diaphragm) against the action of the spring (and against atmospheric pressure), toward the regulator seat to thereby throttle the flow through the regulator until the pressure is reduced to the desired level. One such regulator is described in commonly owned U.S. Pat. No. 5,257,646. Other examples are described in U.S. Pat. Nos. 4,543,985 and 3,890,999.
It is also known to use drain check valves in agricultural irrigation and many other liquid and gas flow systems. See, for example, U.S. Pat. Nos. 4,655,248; 3,874,404; and 4,674,000. A combination check and pressure relief valve is described in U.S. Pat. No. 3,626,977. These valves are usually installed just upstream of respective sprinklers and prevent water from draining out of the sprinklers upon shutdown. In the case of sprinklers installed on a sloping field without drain check valves, all of the water in the lines downstream of the shut-off valve will drain out through the sprinklers at the low end of the field, often flooding that area. The drain check valve is designed to close at some preset pressure that is greater than what would be developed in the line due to elevation. For example, if a line of sprinklers has 32 feet of elevation difference between the head of the line and the tail end of the line, in order to keep the water from draining out of the line, the drain check valve would need to close at 32 feet.times.0.433 p.s.i./ft.=14 p.s.i. There are several drain check valves of this type on the market today, and a typical example of this device is that sold by the assignee of this invention under the name Nelson DCV and the Nelson DCVQC.
The disadvantage of such drain check valves is that they create a pressure loss slightly in excess of the pressure they are designed to check. For example, one drain check valve available from Nelson Irrigation Corp. will check 32 feet of elevation, but will also create 17-18 p.s.i. pressure loss when the sprinkler is operating. This means that the system must be overpressurized by 17-18 p.s.i., which translates into a significant increase in pumping costs.